Solid golf balls, which have good rebound characteristics and small spin amount, are generally approved of or employed by most of general amateur golfers, who regard flight distance as most important characteristics of golf ball. On the other hand, professional golfers and high level golfers regard controllability as most important, followed by soft and good shot feel, and flight performance. Therefore they have mainly employed thread wound golf balls, because they regard controllability as most important and the golf ball has soft and good shot feel. However, it is problem that the thread wound golf ball has a structure, which is easy to apply spin, and it has high spin amount when hit by every golf club, which degrades the flight performance. In order to solve the problem, many solid golf balls, of which the shot feel and flight performance are improved while maintaining good controllability, have been proposed (Japanese Patent Kokai Publication Nos. 332247/1996, 313643/1997, 151320/1999 and the like).
In Japanese Patent Kokai Publications No. 332247/1996, a three-piece solid golf ball which comprises a two-layer structured core composed of an inner core and outer core, and a cover is described. The inner core has a diameter of 25 to 37 mm, has a center hardness in JIS-C hardness of 60 to 85 and has a hardness difference from the center of the inner core to the surface of the inner core is within the range of not more than 4, the outer core has a surface hardness in JIS-C hardness of 75 to 90, and the cover has a flexural modulus of 1,200 to 3,600 kg/cm2. However, since the center hardness of the inner core (center) is high, low spin amount is not sufficiently accomplished when hit by a middle iron club to driver, and the flight distance is not sufficiently obtained.
In Japanese Patent Kokai Publications No. 313643/1997, a three-piece solid golf ball, of which an intermediate layer formed from thermoplastic resin is placed between a core and a cover is described. The core has a center hardness in JIS-C hardness of not more than 75 and has a surface hardness in JIS-C hardness of not more than 85, the surface hardness is higher than the center hardness by 5 to 25, a hardness of the intermediate layer is higher than the surface hardness of the core by less than 10, and a hardness of the cover is higher than the hardness of the intermediate layer. However, since the intermediate layer is formed from thermoplastic resin, it is problem that the rebound characteristics are degraded and the shot feel is hard and poor.
In Japanese Patent Kokai Publication No. 151320/1999, a three-piece solid golf ball which comprises a two-layer structured core composed of an inner core and outer core, and a cover is described. The inner core has a diameter of 15 to 22 mm and Shore D hardness of 40 to 70, the outer core has a JIS-C hardness of 40 to 75, the inner core and outer core are formed from rubber composition, and the cover has a thickness of 0.5 to 3 mm. However, since the center hardness of the inner core (center) is high, low spin amount is not sufficiently accomplished when hit by a middle iron club to driver, and the flight distance is not sufficiently obtained.
In the golf balls described above, since hard and large center is used, the deformation amount at the time of hitting of the golf ball is small, and the spin amount is large and the launch angle is small. Therefore, the flight distance is not sufficiently obtained. In addition, if the flight distance will be improved, shot feel is hard or heavy, and poor. Therefore, a golf ball having sufficient performances has not been obtained.